Laser processing apparatus

ABSTRACT

A laser processing apparatus including a laser device for emitting a first laser beam having a first cross section having a length and a width and an optical system for modifying the first laser beam to produce a second laser beam having a virtual focus. The second laser beam has a second cross section of which length is larger than the length of the first cross section and is constant with propagation of the second laser beam. The apparatus further includes a condenser located after the virtual focus for focusing the second laser beam on a specimen to be treated, wherein said second laser beam is condensed in only a widthwise direction of the cross section, and device for moving the specimen along the widthwise direction. Specifically, laser processing apparatus may include a laser device, a vertical fly-eye lens for homogenizing an intensity along a lengthwise direction of the first cross section, a mirror for directing the laser beam and cylindrical convex lens.

This is a Divisional application of Ser. No. 08/245,587, filed May 18,1994 now abandoned; which itself is a division of Ser. No. 08/081,696,filed Jun. 25, 1993 now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a highly reliable laser annealingprocess suited for use in mass production of semiconductor devices,which enables uniform annealing at high yield. More particularly, thepresent invention provides a laser annealing process of a deposited filmwhose crystallinity had been greatly impaired by the damage it hadreceived through processes such as ion irradiation, ion implantation,and ion doping.

2. Prior Art

At present, methods of lowering of process temperatures in fabricatingsemiconductor devices are extensively studied. The reason for such anactive research for low temperature processes owe partly to the need forfabricating semiconductor elements on an insulator substrate made of,e.g., glass. Laser annealing technology is regarded promising as theultimate low temperature process.

However, conditions for laser annealing are not yet established becauseconventional laser annealing processes were each conducted independentlyunder differing conditions which depend upon the apparatuses and thecoating conditions chosen individually in each process. This has misledand has allowed many to think that the laser annealing technology failsto give results reliable and consistent enough to make the processpractically feasible. An object of the present invention is toestablish, for the first time, the conditions for a laser annealingprocess which yields highly reproducible results.

SUMMARY OF THE INVENTION

In a process for fabricating a semiconductor device, a deposition filmis considerably damaged by processing such as ion irradiation, ionimplantation, and ion doping, and is thereby impaired in crystallinityas to yield an amorphous phase or a like state which is far from beingcalled as a semiconductor. Accordingly, with an aim to use laserannealing in activating such damaged films, the present inventors havestudied extensively how to optimize the conditions of laser annealing.During the study, it has been found that the optimum conditionfluctuates not only by the energy control of the laser beam, but also bythe impurities being incorporated in the film and by the number of pulseshots of the laser beam being applied thereto.

The deposited films to be activated by the process of the presentinvention are those containing, as the principal component, a Group IVelement of the periodic table, e.g., silicon, germanium, an alloy ofsilicon and germanium, or a compound of the Group IV element such assilicon carbide. The deposited film has a thickness of 100 Å to 10,000Å. By taking the light transmission into consideration, it is wellestablished that the laser annealing of such films can be favorablyconducted by applying a laser beam in the short wavelength region, andspecifically, one of 400 nm or shorter.

The process of the present invention comprises the step of:

irradiating laser pulses having a wavelength of 400 nm or shorter andhaving a pulse width of 50 nsec or less to a film comprising a Group IVelement selected from the group consisting of carbon, silicon,germanium, tin and lead and having introduced thereinto an impurity ion,

wherein a transparent film having a thickness of 3 to 300 nm is providedon said film comprising the Group IV element on the way of said laserpulses to said film comprising the Group IV element, an energy density Eof each of said laser pulses in unit of mJ/cm² and the number N of saidlaser pulses satisfy relation log₁₀ N≦-0.02(E-350).

The laser pulses are emitted from a laser selected from the groupconsisting of a KrF excimer laser, an ArF excimer laser, a XeCl excimerlaser and a XeF excimer laser. The introduction of the impurity ion iscarried out by ion irradiation, ion implantation or ion doping. The filmcomprising the Group IV element is provided on an insulating substrate,and the insulating substrate is maintained at a temperature of roomtemperature to 500° C. during the irradiating step.

It had been believed that the sheet resistance can be lowered byapplying a laser beam having an energy density sufficiently high foractivation. In the case of a film containing phosphorus as an impurity,this tendency can be certainly observed. However, in a film containingboron as an impurity, the film undergoes degradation by the irradiationof a laser of such a high energy density. Moreover, it had been takenfor granted that the increase in pulsed shots reduces fluctuation inproperties of the laser annealed films. However, this is not truebecause it was found that the morphology of the coating deteriorateswith increasing number of shots to increase fluctuations in amicroscopic level.

This can be explained by the growth of crystal nuclei within the coatingdue to a laser beam irradiation being applied repeatedly to the film. Asa result, a grain size distribution within a size range of from 0.1 to 1μm appears inside the coating which was previously composed of uniformsized grains. This phenomenon was particularly distinguished when alaser irradiation in the high energy region was applied.

It has been found that the deposited film (i.e. a semiconductor film)must be coated with (covered by) a light-transmitting coating from 3 to300 nm in thickness instead of being exposed to atmosphere. Thelight-transmitting coating is preferably made from silicon oxide orsilicon nitride from the viewpoint that it should transmit laser beam.More preferably, a material mainly comprising silicon oxide is usedbecause, in general, it also serves as the gate dielectric. Needless tosay, the light-transmitting film may be doped with phosphorus or boronwith an aim to passivate the mobile ions. If the film containing a GroupIV element should not be coated with such a light-transmitting coating,it happens that the uniformity is disturbed in a more acceleratedmanner.

It has been found also, that a further smoother (uniform) coating can beobtained by applying pulsed laser beam under a condition set forth aboveand additionally satisfying the following relation:

    log.sub.10 N≦A(E-B)

where, E (mJ/cm²) is the energy density of each of the irradiated laserpulses, and N (shots) is the number of shots of pulsed laser. The valuesfor A and B are dependent on the impurities being incorporated in thecoating. When phosphorus is present as the impurity, -0.02 for A and 350for B are chosen, and an A of -0.02 and B of 300 are selected when boronis included as the impurity.

Similar effect can be attained by using a transparent substrate insteadof the transparent film. That is, a laser process in accordance with thepresent invention comprises the steps of:

introducing an impurity into a semiconductor film provided on atransparent substrate; and

irradiating laser pulses having a wavelength of 400 nm or shorter andhaving a pulse width of 50 nsec or less to said semiconductor filmthrough said transparent substrate,

wherein an energy density E of each of said laser pulses in unit ofmJ/cm² and the number N of said laser pulses satisfy relation log₁₀N≦-0.02(E-350).

FIG. 7(A) shows the introducing step, and FIG. 7(B) shows theirradiating step. Reference numeral 71 designates the transparentsubstrate, and 72 designates the semiconductor film.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic view of a laser annealing apparatus having usedin the embodiments of the present invention;

FIG. 2 is a graph showing the relationship between the sheet resistanceof a silicon film (phosphorus-doped, N-type) obtained by laser annealingaccording to an embodiment of the present invention and the appliedlaser energy density, while changing the repetition times of pulseshots;

FIG. 3 is a graph showing the relationship between the sheet resistanceof a silicon film (phosphorus- and boron-doped, P-type) obtained bylaser annealing according to an embodiment of the present invention andthe applied laser energy density, while changing the repetition times ofpulse shots;

FIG. 4 is a graph showing the relation between the morphology of thesilicon film obtained in an embodiment of the present invention and theapplied laser energy density and the repetition times of the pulseshots;

FIGS. 5A, 5B, and 5C show a concept of an optical system of the laserannealing apparatus having used in the embodiments of the presentinvention;

FIG. 6 shows a laser annealing process in accordance with the presentinvention; and

FIG. 7 shows another laser annealing process in accordance with thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is illustrated in greater detail referring to anon-limiting example below. It should be understood, however, that thepresent invention is not to be construed as being limited thereto.

EXAMPLE

In this EXAMPLE, an impurity is introduced into a film comprising aGroup IV element for imparting one of N-type conductivity and P-typeconductivity thereto, and another impurity is introduced into a portionof the film with a mask for imparting the other one of the N-typeconductivity and P-type conductivity to said portion. In FIG. 1 is shownschematically a laser annealing apparatus having used in the presentexample. A laser beam is generated in a generator 2, amplified in anamplifier 3 after traveling through full reflection mirrors 5 and 6, andthen introduced in an optical system 4 after passing through fullreflection mirrors 7 and 8. The initial laser beam has a rectangularbeam area of about 3×2 cm², but is processed into a long beam having alength of from about 10 to 30 cm and a width of from about 0.1 to 1 cmby the optical system 4. The maximum energy of the laser having passedthrough this optical system was 1,000 mJ/shot.

An optical path in the optical system 4 is illustrated in FIGS. 5A, 5BAND 5C. A laser light incident on the optical system 4 passes through acylindrical concave lens A, a cylindrical convex lens B, a fly-eye lensC provided in a lateral direction and a fly-eye lens D provided in avertical direction. The laser light is changed from an initial gaussdistribution to a rectangular distribution by virtue of the fly-eyelenses C and D. Further, the laser light passes through a cylindricalconvex lenses E and F and is reflected on a mirror G (a mirror 9 inFIG. 1) and is focused on the specimen by a cylindrical lens H.

In this EXAMPLE, distances X₁ and X₂ indicated in FIG. 5 are fixed, anda distance X₃ between a focus I of the lens E and the mirror G,distances X₄ and X₅ are varied to adjust a magnification M and a focallength F. That is,

    M=(X.sub.3 +X.sub.4)/X.sub.5.

    1/F=1/(X.sub.3 +X.sub.4)+1/X.sub.5.

In this EXAMPLE, a total length X₆ of the optical path is about 1.3 m.

The initial beam is modified into a long-shaped one as above to improveprocessability thereof. More specifically, the rectangular beam which isirradiated onto a specimen 11 through the full reflection mirror 9 afterdeparting the optical system 4 has a longer width as compared with thatof the specimen that, as a consequence, the specimen need to be movedonly along one direction. Accordingly, the stage on which the specimenis mounted and the driving apparatus 10 can be made simple structuredthat the maintenance operation therefor can be easily conducted.Furthermore, the alignment operation at setting the specimen can also begreatly simplified.

If a beam having a square cross section were to be employed, on theother hand, it becomes impossible to cover the entire substrate with asingle beam. Accordingly, the specimen should be moved two dimensionallyalong two directions. In such circumstances, however, the drivingapparatus of the stage becomes complicated and the alignment also mustbe done in a two dimensional manner that it involves much difficulty.When the alignment is done manually, in particular, a considerable timeis consumed for this step to greatly reduce the productivity of theentire process. Furthermore, those apparatuses must be fixed on a stabletable 1 such as a vibration proof table.

The specimen used in the example were various types of glass substrates(e.g., a Corning #7059 glass substrate) 100 mm in length and from 100 to300 mm in width. A KrF laser emitting light at a wavelength of 248 nmand at a pulse width of 50 nsec or less, e.g. 30 nsec, was used in theprocess.

A 100 nm thick amorphous silicon film was deposited on a glass substrate61 by plasma assisted CVD (chemical vapor deposition) process. Theresulting film was annealed at 600° C. for 48 hours to obtain acrystallized film, and was patterned to make island-like portions 62 and63 (FIG. 6(A)). Furthermore, a 70 nm thick silicon oxide film (alight-transmitting coating) 64 was deposited thereon by sputtering andthe entire surface of the substrate was doped with phosphorus. Aso-called ion doping process (FIG. 6(B)) was employed in this step usingphosphine (PH₃) as the plasma source and an accelerating voltage of 80kV. Furthermore, a part of the substrate was masked 65 to implant boronby ion doping process (FIG. 6(C)). Diborane (B₂ H₆) was used as theplasma source in this step while accelerating at a voltage of 65 kV.More specifically, phosphorus was implanted (introduced) into the maskedportions through the light-transmitting coating to obtain portion havingrendered N-type conductive, while both phosphorus and boron wereimplanted (introduced) into the unmasked portions through thelight-transmitting coating to result in a portion having rendered P-typeconductive.

Then, laser beam was irradiated to the island-like portions(semiconductor film) while varying the energy density and the number ofpulse shots to effect laser activation. The sheet resistance wasmeasured accordingly and the morphology of the crystallites constitutingthe coating was observed through an optical microscope. The results aresummarized in FIGS. 2 to 4.

FIG. 2 shows a graph which relates the sheet resistance of a siliconfilm having doped with phosphorus ions with the energy density of thelaser beam while also changing the repetition of the pulse shots.Phosphorus was incorporated into the silicon film at a dose of 2×10¹⁵cm⁻². With a laser being operated at an energy density of 200 mJ/cm² orless, a large number of shots were necessary to activate the sheet, yetwith a poor result yielding a high sheet resistance of about 10 kΩ/sq.However, with a laser beam having an energy density of 200 mJ/cm² orhigher, a sufficient activation was realized with a laser operation offrom 1 to 10 shots.

In FIG. 3 is shown the results for laser activating a silicon film dopedwith boron ions at a dose of 4×10¹⁵ cm⁻². In this case again, activationcould be conducted only insufficiently with an energy density of 200mJ/cm² or lower that a large number of pulse shots was required forsufficient activation. With a laser beam operated at an energy densityof from 200 to 300 mJ/cm², a sufficiently low sheet resistance wasobtained with 1 to 10 shots. However, with laser being operated at anenergy density of 300 mJ/cm² or higher, on the other hand, the sheetresistance was reversely elevated. In particular, contrary to the caseof activating with a laser beam energy density of 200 mJ/cm2 or lower,the sheet resistance was elevated with increasing repetition of pulseshots. This phenomenon can be explained by the growth of grain boundarydue to the impaired homogeneity of the film which had resulted byapplying laser irradiation for too many shots.

In a practical process, the laser annealing is applied simultaneously toboth P- and N-type regions as shown in FIG. 6(D). This signifies that alaser beam being irradiated at an energy density of 350 mJ/cm²sufficiently activates the N-type region while impairing the propertiesof the P-type region. Accordingly, in the process according to thepresent example, it is preferred that the laser beam is operated in anenergy density range of from 200 to 300 mJ/cm², and more preferably, ina range of from 250 to 300 mJ/cm². The pulse repetition is preferably inthe range of from 1 to 100 pulses.

As described in the foregoing, the morphology of the deposited film isconsiderably influenced by laser annealing. In fact, the number of pulseshots can be related to the laser beam energy density and the filmmorphology as illustrated in FIG. 4. In FIG. 4, the term "AnnealingPulse" signifies the number of laser beam pulse shots. The solid circlein the figure represents the point at which a change in surfacemorphology was observed on a phosphorus-doped silicon, and the opencircle represents the same on a boron-doped silicon. The upper region onthe right hand side of the figure corresponds to a condition whichyields poor morphology on the surface (rough surface), and the lowerregion on the left hand side of the figure corresponds to that whichyields favorable morphology on the surface (smooth surface). It can beseen from the results that the phosphorus-doped silicon has a strongresistance against laser irradiation. Accordingly, the condition forconducting laser annealing without impairing the surface morphology canbe read to be such which satisfies the relation:

    log.sub.10 N≦A(E-B),

where, E (mJ/cm²) is the energy density of the irradiated laser beam,and N (shots) is the number of shots of pulsed laser. The values for Aand B are A=-0.02 and B=350 in the case phosphorus is incorporated asthe impurity, and are A=-0.02 and B=300 when boron is included as theimpurity.

When the morphology of the deposited film is considerably impaired, thecharacteristic values show large scattering due to the serious dropwhich occurs locally in the properties of silicon. In fact, a scatteringin sheet resistance as high as 20% or even more was observed on asilicon film having a defective morphology (a rough surface). Thisscattering can be removed by satisfying the conditions above and bysetting the laser energy density at a pertinent value.

For instance, when a laser energy density is set at 250 mJ/cm², thepulsed laser beam is shot at a frequency of 10 times or less. If theenergy density is elevated to 280 mJ/cm², the laser beam is preferablyshot at a frequency of from 1 to 3 times. By conducting laser annealingunder such conditions, the sheet resistance could be controlled within afluctuation of 10% or less.

According to the present invention, a highly reliable semiconductor filmhaving low fluctuation in properties was obtained by setting the optimalconditions for laser annealing as described in the foregoing. It can beseen therefore that the process according to the present invention isbeneficial to the semiconductor industry.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

What is claimed is:
 1. A laser processing apparatus for processing asemiconductor film comprising:a laser device for emitting a laser beamhaving a first cross section; a lateral flyeye lens for homogenizing anenergy distribution of said laser beam in a widthwise direction of thecross section; a first cylindrical convex lens for condensing the laserbeam after passing through the lateral flyeye lens in only the widthwisedirection; a mirror for directing said laser beam after passing throughsaid first cylindrical convex lens to said semiconductor film, and asecond cylindrical convex lens for condensing said laser beam in saidwidthwise direction, said second cylindrical convex lens being locatedon an optical path between said mirror and said semiconductor film,wherein a distance X₃ between a focus of said first cylindrical convexlens and the mirror, a distance X₄ between the mirror and said secondcylindrical convex lens, a distance X₅ between said second cylindricalconvex lens and said semiconductor film satisfy the followingconditions:M=(X₃ +X₄)/X₅, where M is a magnification, and 1/F=1/(X₃+X₄)+1/X₅, F is a focal distance of the second cylindrical convex lens;and means for relatively moving said semiconductor film with respect tosaid condensed laser beam.
 2. A laser processing apparatus forprocessing a semiconductor film comprising:a laser device for emitting alaser beam having a cross section; a lateral flyeye lens forhomogenizing an energy distribution of said laser beam in a widthwisedirection; a cylindrical convex lens for condensing the laser beam afterpassing through the lateral flyeye lens in only the widthwise direction;a mirror for directing said laser beam after passing through saidcylindrical convex lens to said semiconductor film; and a condensingmeans for condensing said laser beam only in the widthwise direction inorder to form a condensed laser beam at said semiconductor film, saidcondensing means being located on an optical path between said mirrorand said semiconductor film; and moving means for relatively moving saidsemiconductor film relative to said laser beam in said widthwisedirection, wherein a distance X₃ between a focus of said cylindricalconvex lens and the mirror, a distance X₄ between the mirror and saidcondensing means, a distance X₅ between said condensing means and saidsemiconductor film satisfy the following conditions:1/F=1/(X₃ +X₄)+1/X₅,F is a focal distance of the condensing means.
 3. A laser processingapparatus for processing a semiconductor film comprising:a laser devicefor emitting a laser beam having a cross section; a lateral flyeye lensfor homogenizing an energy distribution of said laser beam in awidthwise direction; a cylindrical convex lens for condensing the laserbeam after passing through the lateral flyeye lens in only the widthwisedirection; a mirror for directing said laser beam after passing throughsaid cylindrical convex lens to said semiconductor film; a condensingmeans for condensing said laser beam only in the widthwise direction,said condensing means being located on an optical path between saidmirror and said semiconductor film; and moving means for relativelymoving said semiconductor film relative to said laser beam in saidwidthwise direction, wherein a distance X₃ between a focus of saidcylindrical convex lens and the mirror, a distance X₄ between the mirrorand said condensing means, a distance X₅ between said condensing meansand said semiconductor film satisfy the following conditions:M=(X₃+X₄)/X₅ where M is a magnification.
 4. A laser processing apparatusaccording to claims 1, 2, or 3 wherein said semiconductor film isannealed by said condensed laser beam.
 5. An apparatus according toclaim 2 or 3 wherein said object includes a semiconductor layer andirradiated portions of said laser beam are modified.
 6. An apparatusaccording to claim 2 or 3 wherein said laser beam is a pulsed laser beamand one site of said object is irradiated with plural number of pulsesof said laser beam.
 7. An apparatus according to claim 2 or 3 whereinthe length of the cross section at said object is 10 cm or greater. 8.An apparatus according to claim 2 or 3 wherein said condensing means isa cylindrical convex lens.
 9. A laser processing apparatus forprocessing a semiconductor film comprising:a laser device for emitting alaser beam having a first cross section; a lateral flyeye lens forhomogenizing an energy distribution of said laser beam in a widthwisedirection of the cross section; a first cylindrical convex lens forcondensing the laser beam after passing through the lateral flyeye lensin only the widthwise direction, a second cylindrical convex lens forcondensing said laser beam having passed through said first cylindricalconvex lens in said widthwise direction, wherein a magnification Msatisfies the following relation:M=(an optical path length between thefocus of said first cylindrical convex lens and the second cylindricalconvex lens)/(an optical path length between the second cylindricalconvex lens and said semiconductor film), and; means for relativelymoving said semiconductor film with respect to the condensed laser beam.10. A laser processing apparatus for processing a semiconductor filmcomprising:a laser device for emitting a laser beam having a first crosssection; a lateral flyeye lens for homogenizing an energy distributionof said laser beam in a widthwise direction of the cross section; afirst cylindrical convex lens for condensing the laser beam afterpassing through the lateral flyeye lens in only the widthwise direction;a second cylindrical convex lens for condensing said laser beam havingpassed through said first cylindrical convex lens in said widthwisedirection, wherein a focal length F of the second cylindrical convexlens satisfies the following relation:1/F=1/(an optical path lengthbetween the focus of the first cylindrical convex lens and the secondcylindrical convex lens)+1/(an optical path length between the secondcylindrical convex lens and said semiconductor film), and means forrelatively moving said semiconductor film with respect to the condensedlaser beam.
 11. A laser processing apparatus for processing asemiconductor film comprising:a laser device for emitting a laser beamhaving a first cross section; a lateral flyeye lens for homogenizing anenergy distribution of said laser beam in a widthwise direction of thecross section; a first cylindrical convex lens for condensing the laserbeam after passing through the lateral flyeye lens in only the widthwisedirection; a second cylindrical convex lens for condensing said laserbeam after passing through said first cylindrical convex lens only insaid widthwise direction, wherein said second cylindrical convex lens isdistant from said first cylindrical convex lens by a distance largerthan a focal length of said first cylindrical convex lens, and means forrelatively moving said semiconductor film with respect to the laser beamcondensed by the second cylindrical convex lens.
 12. A laser processingapparatus for processing a semiconductor film comprising:a laser devicefor emitting a laser beam having a first cross section; a lateral flyeyelens for homogenizing an energy distribution of said laser beam in awidthwise direction of the cross section; a vertical flyeye lens forhomogenizing an energy distribution of said laser beam in a lengthwisedirection of the cross section; a first cylindrical convex lens forcondensing the laser beam after passing through the lateral flyeye lensin only the widthwise direction; a second cylindrical convex lens forcondensing said laser beam after passing through said vertical flyeyelens only in said lengthwise direction, a third cylindrical convex lensfor condensing said laser beam after passing through said first andsecond cylindrical convex lenses only in said widthwise direction; andwherein said third cylindrical convex lens is distant from said firstcylindrical convex lens by a distance larger than a focal length of saidfirst cylindrical convex lens, and means for relatively moving saidsemiconductor film with respect to said condensed laser beam condensedby said third cylindrical convex lens.
 13. An apparatus according toclaims 9, 10, 11, or 12 wherein said semiconductor film is annealed bythe laser beam condensed by said second cylindrical convex lens.